1. Technical Problem Solved by the Invention
For the case of shallow hydrocarbons such as the oil sands in Alberta, Canada, a technique is sought that will map edges, depth extent, and grade (saturation) of a shallow reservoir layer. In this environment, the reservoir layer is manifested in electrical parameters as a resistive anomaly (more resistive than the non-reservoir surroundings) in an already quite resistive background. The background resistivity can range from 50-100 ohm-m whereas the reservoir, depending on quality factors, can vary from 100-1000 ohm-m. In order to be economically accessed by surface mining, the reservoir layer must exist within the upper 70 m of earth. A new technique is disclosed that is able to detect resistivity variations between 50 and 1000 ohm m, at depths of 0 to 100 m from the surface.
2. Previous Techniques and Limitations
Current airborne electromagnetic prospecting involves a helicopter or airplane towing a single receiver and single transmitter over a prospect (FIG. 1). The transmitter broadcasts a specific magnetic signal and the magnetic receiver records the magnetic fields resulting from the source signal interacting with the materials of the earth. In FIG. 1, the transmitter is a magnetic coil attached to a helicopter and flown some 30 m above the ground. The transient electric current in the coil generates a primary magnetic field that penetrates the ground and generates electric currents in the conductive sediments. As a result, a secondary magnetic field is generated and recorded by a receiver comprising conductive coils depicted in the drawing. Information about the sediments is captured by the secondary field. This method has predominantly been employed for the identification of precious metal deposits and groundwater characterization. Both of these applications require the detection of a conductive anomaly (0.01-1 ohm m) within a resistive background (>100 ohm m). While this technique allows for the fast collection of data over a broad area, it is limited to one fixed source-receiver offset, and it is also limited by the fact that it must necessarily be vertically distant from the object it is intended to detect. For these reasons, it is not suitable for detecting the hydrocarbon target previously described.
The magnetic signal arising from the relatively resistive hydrocarbon is very weak and likely obscured within the noise level of a receiver flown above ground. FIG. 3 shows a synthetic sensitivity study, performed by the present inventors, where the altitude of the source and receiver was chosen to be comparable with the flight conditions at treetop level. FIG. 3 shows, on the left, sensitivity as a function of source frequency and source-receiver offset, for a resistive target. On the right, FIG. 3 shows sensitivity for a conductive target. The thin solid contour lines in the sensitivity plots (resistive target on the left, conductive target on the right) enclose regions of detectability in the data, i.e., a signal-to-noise ratio (SNR) greater than 1. In both sensitivity plots, the bold dashed vertical line shows the data coverage of current technology, i.e. a single offset and multiple frequencies. FIG. 3 also shows the resistivity models used to generate the synthetic data studies. Thus, it can be seen from FIG. 3 that the current fully airborne electromagnetic prospecting is good for detecting metallic ores (conductive bodies), which is what it was originally designed for, but not good, regardless of frequency or offset, for detecting hydrocarbons (resistive bodies).
Exacerbating the distant-receiver problem is the fact that there is a limited range of frequencies that are both able to be transmitted with significant power, and able to invoke strong enough secondary-field anomaly from the reservoir to be detectable above the magnetic field resulting from the background geology. Geophysical inverse problems (inverting the geophysical data to infer the subsurface physical property model that gave rise to the data) often suffer from the problem of non-uniqueness, the electromagnetic problem especially so. The fewer independent geophysical observations we have, the larger the uncertainty of the recovered image of the subsurface will be. In this case, the narrower the frequency range of sensitivity to the reservoir, the weaker the constraints on the pertinent parameters of the reservoir target (e.g. aerial distribution, thickness, resistivity, depth of burial).
In order to address some of the issues, a different approach was considered by some research groups. Some relevant publications include:
U.S. Pat. No. 5,610,523, 1997 to P. J. Elliot, “Comparison of data from airborne, semi-airborne, and ground electromagnetic systems;”
R. S. Smith, et al., “Method and apparatus of interrogating a volume of material beneath the ground including an airborne vehicle with a detector being synchronized with a generator in a ground loop,” Geophysics 66, 1379-1385 (2001); and
T. Mogi, et al., “Grounded electrical-source airborne transient electromagnetic (GREATEM) survey of Mount Bandai, north-eastern Japan,” Exploration Geophysics 40(1), 1-7 Published online: 27 Feb. 2009.
Elliot's patent proposes a method of interrogating a volume of underground material located beneath a grounded loop transmitter whose transient electromagnetic signal is picked up by a receiver attached to an aircraft. Smith et al. considered an experimental semi-airborne system with a source loop placed on the ground and an airborne receiver, and investigated how the signal-to-noise level compares with the case of an earth-bounded survey and an airborne one. Mogi et al. used the semi-airborne technology with a grounded transient electromagnetic source and an airborne receiver for investigating volcanic structures.
All three of the above publications propose a semi-airborne survey method that places the source on the ground while the receiver is attached to an aircraft. Although this approach addresses some of the shortcomings of the existing technology, in particular the acquisition of multiple-offset data, the low signal-to-noise ratio for resistive targets remains a problem. With the sensitive receiver placed on a moving platform, much higher noise is generated through motional induction, than would be experienced in a stationary receiver on the ground, relative to the small signal from the reservoir. Conversely, the anomalous currents that might be induced through motion of the transmitter would be orders of magnitude smaller than the known current that drives it. In addition, the existing semi-airborne approach, by pinning a singular transmitter to the ground does not allow for the economical collection of data from multiple, sequential source locations. There is a need for a technique that mitigates the problems of weak reservoir signal relative to receiver noise and lack of constraints on the data inversion. The present inventive method satisfies this need.